The invention relates to a method for imaging a printing form with an imaging device which includes a laser diode bar having a number n of individually controllable laser diodes, each of which are assigned to an imaging channel, whereby the imaging spots of the imaging channels are arranged essentially in a row on the printing form. Furthermore, the invention relates to a method for imaging a printing form with a number b of imaging devices each of which includes a laser diode bar, each of which has a number n of individually controllable laser diodes, whereby each laser diode is assigned to an imaging channel, and the imaging spots of the imaging channels of the number b of laser diode bars are arranged essentially in a row on the printing form.
Several imaging channels, especially those equipped with laser diodes, are often used at parallel time intervals in printing form exposure units, or printing units of printing machines containing imaging devices (so-called Direct Imaging Printing Units) in order to efficiently reduce the imaging time for the exposure of the two-dimensional surface of the printing form. If an redundant-free imaging method is used, i.e., if the imaging channels are shifted across the two-dimensional surface of the printing form in such a way that the location of each printing dot to be placed by an imaging channel is passed exactly once, the imaging time for the entire surface to be imaged with an imaging device with n imaging channels is reduced to (1/n) of the time. An additional reduction of time can also be efficiently achieved with the parallel use of b imaging units each of which exposes sections of the printing form in a redundant-free manner analogously to the procedure described above. In this case, the imaging time for the entire surface to be imaged is reduced to (1/b) of the time, more exactly, with the use of b imaging devices with n imaging channels, to (1/(bn)) of the time.
The substantial reduction of imaging time by means of redundant-free parallelization thus strongly depends on the number of the available (capable of being activated) or used imaging channels.
In order to pass, in a redundant free manner, the locations of a two-dimensional surface of a printing form on which printing dots are to be set with a number of imaging channels (regardless whether arranged on one or more imaging devices), certain feed rules are to be observed for the passing of locations to be imaged in a time pre-arranged step to locations to be imaged in a step next in time. These feed rules particularly are to be strictly met if an imaging step using n imaging channels sets n printing dots in locations that are not densely positioned on the printing form, i.e., the distance between them is not the minimum printing dot distance p (typically 10 micrometers). In order to achieve dense imaging, printing dots are set between already imaged printing dots in an imaging step next in time. This procedure also is known by as the term interleave-process (interleaving). For example, German Patent Application No. DE 100 31 915 A1 characterizes an interleave procedure for the exposure of printing forms: at a given minimum printing dot distance p and for a number of n imaging channels on a setting line at even distances to one another, the neighboring printing dots of which have a distance a on the printing form, which is a multiple of the minimum printing dot distance p, a redundant-free feed is ensured for the distance (np) in the direction of the setting line if the natural numbers n and (a/p) are prime.
In this regard, it should be explained that the two-dimensional surface of the printing form to be imaged is typically passed over rapidly by the imaging channels in a first direction, and slowly in a second direction, which is linearly independently of, preferably perpendicular, to the first direction. In this case, the setting line will not be positioned parallel to the rapid first direction, but can be tilted toward the slow second direction at an angle that is not zero. A low printing dot distance can be achieved by this tilt by the cosine factor of the angle (projection). Preferably, the setting line will be positioned perpendicularly in the rapid first direction. The printing dots of the imaging channels can also be set on the setting line using tripping times delayed relative to one another, between which the relative movement is continued between the imaging device and the printing form. Delayed tripping times are helpful, for example, for correcting geometrical errors of the imaging device structure.
The performance of a redundant-free interleave process according to document DE 100 31 915 A1 is critically dependent on the fact that n imaging channels are also available, i.e. can be activated, at even distances on a setting line. As the strategy to be followed in case of failure or malfunction of an imaging channel this document recommends using the largest remaining contiguous section of the imaging channels at even distances, if non-imaged strips on the printing form are to be avoided, and an equally good imaging quality is to be ensured. It is obvious that in order to realize a redundant-free interleave process according to the document, a number of the imaging channels of the remaining contiguous section must be selected that is prime with respect to the distance multiple (a/p). In following this strategy, any failures or malfunction of further imaging channels results in very short sections of the originally n parallel imaging channels. As a consequence, the imaging time substantially increases with the decrease of the still available parallelization. For example, in the unfavorable case of a failure of one imaging channel each in the center of the largest contiguous section on the setting line, the imaging time each increases to twice as long, i.e., a multiple of the originally parallelized imaging time in the case of several failures. This is completely unacceptable in practice.
The failure or malfunction of a laser diode is generally especially critical with the use of laser diode bars in imaging devices if exactly one laser diode is assigned to each imaging channel, because in order to reestablish the original functionality replacement of the entire laser diode bar is necessary. For economical reasons alone this is not feasible, because the other laser diodes on the bar are generally still functional, and the laser diode bar as a whole has not completely malfunctioned.
U.S. Pat. No. 6,181,362 B1 recommends assigning two laser diodes for each imaging channel on the laser diode bar. For imaging a printing form, one laser diode each is used per imaging channel. In the case of failure of the first laser diode in an imaging channel, the second laser diode is used in its place. However, the document leaves open how to proceed if the redundant laser diodes of an imaging channel fail at the same time.
As an alternative, U.S. Pat. No. 6,252,622 B1 recommends assigning a first laser diode on a first laser diode bar, and a second laser diode on a second laser diode bar for each imaging channel. For imaging a printing form, one laser diode of one of the two laser diode bars is used per imaging channel. In case of a failure of the first laser diode on the first laser diode bar in an imaging channel, the second laser diode on the second laser diode bar is used in its place. However, the document leaves open how to proceed if the redundant laser diode of an imaging channel fails at the same time.
The solutions of both documents U.S. Pat. No. 6,181,362 B1, and U.S. Pat. No. 6,252,622 B1 have in common that, roughly speaking, a replacement diode is provided for each imaging channel in the case of a malfunction. As a consequence, this method is cost intensive. From the start, twice the number of laser diodes is required in order to ensure a safe strategy. A priori, a multitude of replacement diodes are generally not necessary in practice. Both documents fail to offer any principal solution for the problem of how to proceed in case one or several imaging channels fail.